1. Field of the Invention
The present invention relates to a combination antenna and, more particularly, to a combination antenna having a Global Positioning System antenna (GPS) and a VHF communication antenna co-located within a common structure and operable simultaneously.
2. Description of the Related Art
Conventionally, antennas are designed to service a single operating band, utilizing a single operating mode. Thus, multiplicity of systems operable at different bands necessarily leads to the profusion of antennas on the host platform. Because each system has both installation and maintenance costs associated with it, it is desirable, whenever possible, to combine and integrate the various system components.
A carefully designed integration is particularly beneficial when a new system is added to an already operating platform. In such cases, the platform needs to be removed from service and, generally, the cost of adding the new system are governed by the off-service time. Moreover, the addition of a new antenna necessitates careful designing to prevent interference with the existing antennas. Thus, one must carefully select the location on the platform for the installation of the appropriate antennas, and provide for effective shielding where necessary. One must also take into account compliance with appropriate government safety rules and other certification requirements when applicable, such as an FAA airworthiness certification.
One way to integrate the various systems is to combine several antennas into a single structure. Such a combination is disclosed, for example, in U.S. Pat. Nos. 4,030,100 and 4,329,690. Both of these patents relate to marine vessel applications that include a GPS antenna in combination with other antennas. In order to provide a clear view of the top hemisphere, these references teach positioning the GPS antenna at the top of the arrangement, so as to be unobstructed by the other antennas. Similarly, in order to prevent interference, elaborate structural shielding is described.
Since aerodynamic, weight, and space considerations are of utmost importance in aircraft applications, the requirements of having the GPS antenna physically shielded and positioned on top of the structure is of major disadvantage. The GPS antenna is of considerable thickness, and aerodynamic considerations would, therefore, require it to be mountable as close as possible to the aircraft fuselage. Similarly, the structural shielding requires additional space and adds burdensome weight. It is therefore desirable to provide for a combination antenna having the GPS antenna situated at the bottom of the structure and to dispense with the physical shielding.